Thermal printing selection circuitry



Sept. 16, 1969 Filed Oct. 2, 1967 FIG. 2

R. c. CADY, JR

THERMAL PRINTING SELECTION CIRCUITRY GROUP FOUR GROUP THREE GROUP TWO Y GROUP ONE CURRENT 2 Sheets-Sheet 1 VOLTAGE INVENTOR RICHARD C. CADY, JR.

HIS ATTORNEYS Sept. 16, 1969 R. c. CADY, JR

THERMAL PRINTING SELECTION CIRCUITRY 2 Sheets-Sheet 2 03% aDOmQ uZO QDOKO r Ill Ill 1/ Filed 001.. 2. 1967 INVENTOR RICHARD C. 'CADY, JR.

HIS ATTORNEYS United States Patent 3,467,810 THERMAL PRINTING SELECTION CIRCUITRY Richard C. Cady, Jr., Dayton, Ohio, assignor to The National Cash Register Company, Dayton, Ohio, a corporation of Maryland Filed Oct. 2, 1967, Ser. No. 672,323 Int. Cl. G01d 15/10; Hb [/02 US. Cl. 219216 28 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION In thermal printing systems, it is desirable to print a number of characters on a thermal printing record material without movement of the thermal printing record material until a complete row of information, in the form of a row of dots, has been printed. If all of the thermal printing elements associated with a row of information must be individually energized and then de-energized before another thermal printing element can be energized, the print cycle time is greatly extended. The present invention provides a thermal printing circuit which successively energizes selected thermal printing elements and then simultaneously de-energizes all of the thermal printing elements, thereby reducing the thermal print cycle time considerably. In addition, the thermal printing circuitry of the present invention is simple and reliable and may employ a number of alternate types of switching devices.

SUMMARY OF THE INVENTION The present invention comprises a thermal printing selection circuit which enables a row of thermal printing elements to be selectively energized and then de-energized simultaneously, so that all of the selected thermal printing elements may simultaneously print information, in the form of a row of dots, on a thermal record mate rial, thereby greatly reducing the required thermal print cycle time.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 shows a voltage-current characteristic for a silicon controlled-recitifier.

FIGURE 2 is a chematic diagram of a silicon controlled-rectifier embodiment of the present invention.

FIGURE 3 is a voltage-current characteristic for a two-terminal switching device which may be employed with the present invention.

FIGURE 4 is a schematic diagram of an embodiment of the present invention employing two-terminal switching devices which have the voltage-current characteristic of FIGURE 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the present invention, a plurality of electronic switching devices are employed, and each switching device is connected in series with an electrically resistive thermal Patented Sept. 16, 1969 printing element. The switching devices of the present invention have the voltage-current characteristic such that the switching device is in an initial high-impedance state when the applied voltage to the switching device is less than a predetermined value, and is in a low-impedance state after the applied voltage to the switching device has exceeded the predetermined value and a minimum hold ing current is maintained through the device. One side of each switching device-thermal printing element combination is connected to a power supply bus which is energized only during the print cycle. The other side of each switching device-thermal printing element combination is connected in common with a number of other switching device-thermal printing element combinations. The common junction point of each group of switching device-thermal printing element combinations is coupled to a group selectio transistor, which has a Zener diode coupled across it, and to a resistor that is coupled to a power supply. A plurality of element selection lines, equal in number to the number of switching device-thermal printing element combinations in a group, are each connected to a triggering means and-to one of the switching devices of each group.

Initially, all of the switching devices in the thermal printing system are biased into a high-impedance state. When a particular group selection transistor is selectively saturated, and the power supply bus is also energized, the operating points of all of the switching devices which are coupled to that group selection transistor are forced beyond their forWard-breakover voltage regions, and these devices, therefore, switch into their low-impedance state. The saturated group selection transistor is then turned off, and another group of thermal printing elements may then be selectively energized in the manner described. Current, however, continues to flow through the previously selected switching devices and the Zener diode of the previously selected group, even though the group selection transistor is cut-off, as long as the current flowing through the selected switching devices is greater than the required minimum holding current. At the end of the complete thermal print cycle, which occurs after all of the group selection transistors have been consecutively energized, the voltage supplied to the power supply bus is removed, and the current through all of the previously selected switching devices falls the minimum holding current value, causing all of the previously selected switching devices to revert to their high-impedance state.

One of the disclosed embodiments employs silicon controlled-rectifiers as the switching device, and in that embodiment the element selection lines are coupled to the gate electrodes of the silicon controlled-rectifiers. Another disclosed embodiment employs two-terminal switching devices which have a voltage-current characteristic that is similar to the voltage-current characteristic of a silicon controlled-rectifier. In that embodiment, the power supply bus is divided into a number of power supply lines which are each coupled through diodes to one switching device-thermal printing element combination of each group. The element selection lines from the triggering voltage means in this embodiment are connected so that when a selection line is energized, the coupling diode in the associated power supply line is reverse-biased, and the triggering voltage from the triggering voltage means, which is higher than the supply voltage on the power supply bus, is applied to the switching device.

FIGURE 1 shows a typical voltage-current characteristic of a silicon controlled-rectifier. When the silicon controlled-rectifier is in the high impedance state 10, an increase in voltage will produce a very small increase in current. For example, the operating point may move from point A to point B of FIGURE 1. When the voltage applied to the silicon controlled-rectifier is increased past point C of FIGURE 1, which corresponds to the forwandbreakover voltage, the silicon controlled-rectifier switches to a low-impedance state which is represented by the portion 12 of the voltage-current characteristic of FIGURE 1. The increase in current which occurs when the forwardbreakover voltage is exceeded is due to avalanche multiplication, and the silicon controlled-rectifier will remain in the low-impedance state 12 as long as the current through the silicon controlled-rectifier is greater than a minimum holding current 16. When the current through the silicon controlled-rectifier falls below the minimum holding current 16, the silicon controlled-rectifier will revert to its high-impedance state When the silicon controlled-rectifier is reverse-biased, it essentially resembles two reverse biased PN junctions in series, and it exhibits a reverse characteristic that is very similar to ordinary reverse-biased rectifiers. As the gate current that is supplied to a silicon controlled-rectifier is increased, the voltage-current characteristic of the silicon controlledrectifier is altered, so that the forward-breakover voltage is reduced, as shown by the curves 19 and 21 of FIGURE 1. Therefore, as the gate voltage is increased, a smaller amount of trigger power is required to trigger the silicon controlled-rectifier into the low-impedance state, where it can provide a large amount of power to its series-connected thermal printing element. Once the gate electrode has been supplied with the necessary triggering pulse and the silicon controlled-rectifier has switched to the low-impedance state, the gate electrode no longer has any control over the silicon controlled-rectifier, and the silicon controlled-rectifier must be turned off by reducing the current flowing through the device below the minimum holding current 16. Since the silicon controlled-rectifier is a current-triggered device instead of a voltage-triggered device, a low-impedance voltage source is required to trigger the silicon controlled-rectifier.

In the embodiment of FIGURE 2, a thermal printer is formed of a number of thermal printing elements 24, each of which is connected to a separate silicon controlledrectifier 22. A thermal printing record material (not shown) is placed adjacent to the thermal printing elements 24. The construction of suitable thermal printing elements is described in US. Patent No. 3,161,457, issued Dec. 15, 1964, on the application of Hans Schroeder, William H. Puterbaugh, and Robert C. Meckstroth. In the present invention, the printing head may consist of a single elongated substrate with all of the thermal printing elements arranged side by side along the substrate, if desired. The thermal printing elements 24 are electrically resistive elements which are made of a material, such as tin oxide, that heats when conducting an electrical current. Printing then occurs on a thermally sensitive record material which is placed adjacent to an energized thermal printing element. A suitable thermally sensitive record material is described in U.S. Patent No. 3,293,055, issued Dec. 20, 1966, on the application of Henry H. Baum.

The anode electrodes 26 of the silicon controlled-rectifiers 22 are connected to one terminal of the thermal printing element 24, and the other terminal of the thermal printing element 24 is connected to a common power supply bus 56. The gate electrodes 28 of the silicon controlled-rectifiers 22 are coupled to the element selection lines 58. The thermal printing elements 24 and the silicon controlled-rectifiers 22 are arranged into a number of groups, so that each element selection line 58 is connected to the gate electrode 28 of one silicon controlledrectifier 22 in each group. The cathode electrodes 30 of all of the silicon controlled-rectifiers 22 in a group are joined together and are connected to the collector electrode 38 of an associated group selection transistor 20. The emitter electrodes 36 of the group selection transistors are grounded, and Zener diodes 32 are coupled across the collector electrodes 38 and the emitter electrodes 36. In the illustrated embodiment, the group selection transistors 20 are NPN transistors, although, of course, PNP transistors can be employed if the proper voltage polarities are determined in the manner known to those skilled in the art. The collector electrodes 38 are coupled to a positive voltage supply through the resistors 34, which insure that sufficient current is supplied through the Zener diodes 32 when the associated group selection transistor 20 is cut-01f to develop the Zener voltage.

Initially, all of the group selection transistors 20 are biased into a non-conducting cut-off state, and, therefore, the collector electrodes 38 of these transistors are maintained at the Zener diode voltage level of the associated Zener diode 32. If the power supply bus 56 is energized by a positive supply voltage, all of the silicon controlledrectifiers 22 will be biased to point A on the forwardblocking portion 10 of the voltage-current characteristic of FIGURE 1. In this condition, if a triggering voltage is supplied to the element selection lines 58, from a triggering voltage means (not shown), the operating point of the silicon controlled-rectifiers will move from point A to point B. and consequently no substantial change in current through the silicon controlled-rectifiers will occur. Now, however, if a positive selection voltage is also supplied to the base 42 of the group selection transistor 20 of group 1, for example, this transistor will be saturated, and its collector electrode 38 will be at a potential which is very close to ground. The Zener diode 32 is then effectively shorted out, and, since the power supply bus 56 is energized by a positive voltage, the operating points of all of the silicon controlled-rectifiers 22 of group 1 which have a triggering current supplied to their gate electrodes 28 will now pass into their forwardbreakover regions, causing these selected silicon controlled-rectifiers to switch into their low-impedance state 12. When a selected silicon controlled-rectifier has switched into its low-impedance state 12, it allows a large amount of current to flow through the associated thermal printing element 24, thus providing sufiicient heat to print on a thermal record material that is positioned in the vicinity of the thermal printing elements 24. The point D in FIGURE 1 represents the low-impedance state of a selected silicon controlled-rectifier 22, which is determined by the intersection of the load line 14 and the voltage-current characteristic 12. The triggering signals that are applied to the element selection lines 58 are preferably simultaneous signals, although consecutive triggering signals may be employed if desired.

Following energization of all of the desired sicilon controlled-rectifiers in group 1, the selection voltage on the element selection lines 58 is removed, and the positive voltage signal that was applied to the base 42 of the group selection transistor 20 of group 1 is also removed. Current, however, will continue to flow from the power supply bus 56 through the selected heat printing elements 24, the selected silicon controlled-rectifiers 22 of group 1 and the associated Zener diode 32 to ground. The positive voltage supply that is connected to the power supply bus 56 remains energized, and the group selection transistor for group 2 is next energized by the application of a positive selection signal to its base 42. The selected silicon controlled-rectifiers of group 2 are then energized in a manner that is identical to that described for group 1. The selection transistor for group 2 is then turned off, but current also continues to flow through the selected thermal printing elements of group 2.

In the manner described, all of the group selection transistors are consecutively energized, and the associated silicon controlled-rectifier elements 22 are selectively energized during the time that the associated group selection transistor 20 is energized. The time during which the silicon controlled-rectifiers 22 remain energized is considerably greater than the time required to turn on a silicon controlled-rectifier, and, therefore, all of the silicon controlled-rectifiers 22 of the thermal printing system may be considered as being effectively simultaneously energized. Following the energization of the last group of silicon controlled-rectifiers of the system, the voltage on the power supply bus 56 is returned to a ground potential level, and the current flowing through all of the silicon controlled-rectifiers then decreases below the minimum holding current 16, causing the previously selected silicon controlled-rectifiers to switch back to their high-impedance state 10. In the described embodiment ofFIGURE 2, it should be noted that the Zener voltage of the Zener diodes 32 must be equal to or more positive than the voltage on the element selection lines 58 in order to insure reliable operation.

FIGURE 3 is a voltage-current characteristic for a two-terminal glass body or semiconductive switching device. Examples of such devices are described in US. Patent No. 3,241,009, issued Mar. 15, 1966, on the application of Jacob F. Dewald, William R. Northover, and Arthur D. Pearson; and US. Patent No. 3,271,591, issued Sept. 6, 1966, on the application of Stanford R. Ovshinsky. These devices are composed of either glass or semiconductive material and have a voltage-current characteristic which is similar to the voltage-current characteristic of a silicon controlled-rectifier. A high-impedance state a low-impedance state 12', and a forward-breakover point C are found in the voltage-current characteristics of these switching devices, as shown in FIGURE 3.

FIGURE 4 is a schematic drawing that shows an embodiment of the thermal printing system of the present invention which is constructed by employing twoterminal glass or semiconductive switching devices which are similar to or identical with the switching devices of the Dewald et a1. and Ovshinsky US. patents. In the schematic of FIGURE 4, the two-terminal switching devices 62 are coupled in series with the thermal printing elements 64. The two-terminal switching device-thermal printing element combinations of this embodiment each have one terminal of the thermal printing elements 64 connected in common with the collector terminal 80 of an associated group selection transistor 68. The power supply bus 66 is divided into a number of separate power lines 70. Each of the power supply lines 70 is coupled to the power supply bus 66 through a semi-conductor diode 72, and is also connected to one two-terminal switching device 62 of each group of thermal switching devices. A plurality of element selection lines 76, equal in number to the number of two-terminal switching devices 62 in each group, are also coupled so that one element selection line 76 is connected to each of the power supply lines 70. The triggering voltage from the triggering voltage means (not shown) which is supplied to the element selection lines 76 has a larger magnitude than the supply voltage which is supplied to the power supply bus 66. Thus, when a positive triggering voltage is supplied to an element selection line 76, the associated semiconductor diode 72 will be reverse-biased, thereby isolating the triggering voltage means from the power supply bus 66.

In a manner similar to that described for the silicon controlled-rectifier embodiment of FIGURES 1 and 2, when all of the group selection transistors 68 are cutoff, then all of the two-terminal switching devices 62 are biased into a high-impedance state 10, and, when a triggering voltage is applied to a selected element selection line 76, the associated two-terminal switching device 62 that is coupled to a group selection transistor which has been driven into saturation will have a voltage applied across it which exceeds the foiward-breakover voltage of the switching device, causing it to switch into a lowimpedance state 12'. The magnitude of the triggering voltage minus the Zener diode voltage must exceed the forward-breaker voltage of the switching devices 62 in this embodiment. Once the two-terminal switching device 62 has been switched into a low-impedance state 12', the triggering voltage that was applied to the associated element selection line 76 may be removed, and the associated group selection transistor 68 may be turned off. In this condition, the voltage of the common collector electrode is established at the Zener voltage of the Zener diode 78 by the current flowing through the switching devices 62, and the selected two-terminal switching devices 62 will remain in their conductive state as long as a minimum electrical current flow is maintained through them, even though the group selection transistor 68 is cut-oil. The selected two-terminal switching devices 62 of the embodiment of FIGURE 4 remain in an energized condition until all of the groups of thermal printing elements 64 have been consecutively selectively energized. The following selected energization of the last group of thermal printing elements 64 of the system the voltage app-lied to the power supply bus 66 falls to a ground level, and all of the two-terminal switching devices switch back to their high-impedance state 10'.

It will be recognized by those skilled in the art that, although three-terminal silicon controlled-rectifiers and two-terminal glass and semiconductive switching devices have been described as being usable in the present invention, many other equivalent two-terminal, three-terminal, and even four-terminal or more switching devices may be readily adapted for use in the present invention, based on the teachings of the present disclosure. In addition, although each group of switching device-thermal printing element combinations in the embodiments of FIGURES 2 and 4 have an identical number of such combinations, it is apparent that this is not an essential limitation. Light activation in particular is considered as being included Within the scope of the present invention, since the necessary triggering signals for both of the described embodiments may be alternately achieved by employing light triggering sources instead of voltage triggering sources. A non-exclusive listing of three-ter minal switching devices which may be employed in the present invention includes silicon unilateral switches, bidirectional triode thyristors, silicon bilateral switches, silicon controlled switches, light-activated semiconductor controlled-rectifiers, thyratrons, and light-activated silicon controlled switches. A non-exclusive listing of two-terminal switching devices which may be used in the present invention includes trigger diacs, bi-directional diode thyristors, and light-activated switches. Representative examples of voltage-current characteristics of the abovementioned devices may be found in General Electric Companys SCR manual, fourth edition.

What is claimed is:

1. An electrical circuit comprising:

(a) a plurality of switching means-electrically resistive combinations that each have a first junction and a second junction and are each assigned to one group of a plurality of groups of said combinations, each combination comprising:

(i) a triggerable switching means that is reversibly switchable between an initial high-impedance state and a low-impedance state, and

(ii) an electrically resistive element that is connected in series with the switching means, and

(b) means to successively apply a plurality of sets of selective triggering signals to said combinations, the plurality of sets of selective triggering signals being equal in number to the number of groups of said combinations, and

(0) means to apply a supply voltage to the second junction of at least those combinations which have no simultaneous triggering signal applied thereto during the period of time that the plurality of sets of triggering signals are being applied to the switching means, and

(d) means to apply a first potential to the first junctions of the combinations of each group in an individual manner and to simultaneously apply a second potential to the first junctions of all of the combinations of the other groups, the first potential being applied to the first junctions of the combinations of a group during the time that a set of selective triggering signals are being applied to the switching means, the means to apply the first and the second potentials being constructed so as to provide a current path that allows at least a predetermined minimum amount of electrical current to flow through the switching means due to the application of the supply voltage pulse, the application of a selective triggering signal being effective to switch a selected switching means in the group which has the first potential applied thereto from an initial high-impedance state to a low-impedance state, the termination of the supply voltage pulse being efiective to switch the previously-selected switching means back to the high-impedance state.

2. The invention of claim 1 wherein the electrically resistive elements are thermal printing elements.

3. The invention of claim 1 wherein the means to apply the first and the second potentials comprises:

(a) a plurality of group selection transistors each having a control electrode and a first and a second load electrode, each transistor having its first load connected to all of the first junctions of one group of saidcombinations, and

(b) a selective energization signal means that is coupled to apply an energization signal which is effective to saturate a group selection transistor to the control electrodes of each of the group selection transistors in a successive and individual manner, and

(c) a plurality of voltage-breakdown means, each voltage-breakdown means being connected across the second and the first load electrodes of a separate group selection transistor, so as to provide a substantially constant voltage across the transistor when the transistor is cut-off, the voltage-breakdown means having a voltage-current characteristic such that the current flowing through an associated switching means predominantly flows through the transistor when the transistor is saturated and predominantly flows through the voltage-breakdown means when the transistor is cut-off, and

(d) a resistor connected between the first load electrode of each group selection transistor and a power supply.

4. The invention of claim 3 wherein the voltage-breakdown means is a Zener diode.

5. The invention of claim 4 wherein the electrically resistive elements are thermal printing elements.

6. An electrical circuit comprising:

(a) a plurality of triggerable switching means that are reversibly switchable between an initial high-impedance state and a low-impedance state which have a first load electrode, a second load electrode, and a control electrode, each switching means being assigned to one group of a plurality of groups of switching means-electrically resistive element combinations, and

(b) a plurality of electrically resistive elements, each having a first terminal that is connected to the first load electrode of a switching means and a second terminal that is connected in common with a second terminal with all of the other electrically resistive elements, and

() means to successively apply a plurality of sets of selective triggering signals to the control electrodes of the switching means, the plurality of sets of selective triggering signals being equal in number to the number of groups of said combinations, and

((1) means to apply a supply voltage pulse to the common second terminals of the electrically resistive elements during the time that the plurality of sets of triggering signals are being applied to the switching means, and

(e) means to apply a first potential to the second load electrodes of the switching means of each group of said combinations in an individual manner and to simultaneously apply a second potential to the second load electrodes of all of the switching means of the other groups of said combinations, the first potential being applied to the second load electrodes of the switching means of a group of said combinations during the time that a set of selective triggering signals'are being. applied to the control electrodes of the switching means, the means to apply the first and the second potentials being constructed so as to provide a current path that allows at least a predetermined minimum amount of electrical current to flow through the switching means due to the application of the supply voltage pulse, the application of a selective triggering signal being effective to switch a selective switching means in the group of said combinations which has the first po- 1 tential applied thereto from an'initial high-impedance state to a low-impedance state, the termination of the supply voltage pulse being effective to switch the previously selected switching means back to the high-impedance state. I 7. The invention of claim 6' wherein the electrically resistive elements are thermal printing elements.

8. The invention of claim 6 wherein the switchin means are controlled-rectifiers.

9. The invention of claim 6 wherein the means to apply the first and the second potentials comprises:

(a) a plurality of group selection transistors, each having a control electride and a first and a second load electrodes, each transistor having its first load electrode connected to all of the first junctions of one group of said combinations, and (b) a selective energization signal means that is coupled to supply an energizing signal which is effec tive to saturate a group selection transistor to the control electrodes of each of the group selection transistors in a successive and individual manner, and (c) a plurality of voltage-breakdown means, each voltage-breakdown means being connected across the second and the first load electrodes of a separate group selection transistor so as to provide a substantially constant voltage across the transistor when the transistor is cut-off, the voltage-breakdown means having a voltage-current characteristic such that the current flowing through an associated switching means predominantly flows through the transistor when the transistor is saturated and predominantly flows through the voltage-breakdown means when the transistor is cut-off, and (d) a resistor connected between the first loadelectrode of each group selection transistor and a power supply. 10. The invention of claim 9 wherein the voltage-breakdown means is a Zener diode that has a breakdown voltage that is equal to or greater in magnitude than the magnitude of the voltage of the triggering signal.

11. The invention of claim 8 wherein the means to apply the first and the second potentials comprises:

(a) a plurality of group selection transistors each having a control electrode and a first and a second load electrodes, each transistor having its first load electrode connected to all of the first junctions of one group of said combinations, and

(b) a selective energization signal means that is coupled to apply an energization signal which is efiective to saturate a group selection transistor to the control electrodes of each of the group selection transistors in a successive and individual manner, and

(c) a plurality of voltage-breakdown means, each voltage-breakdown means being connected across the second and the first load electrodes of a separate group selection transistor so as to provide a substantially constant voltage across the transitor when the transistor is cut-ofi, the voltage-breakdown means having a voltage-current characteristic such that the current fiowing through an associated switching means predominantly fiows through the transistor when the transistor is saturated and predominantly flows through the voltage-breakdown means when the transistor is cut-off, and

(d) a resistor connected between thefirst load electrode of each group selection transistor and a power supply.

12. The invention of claim 11 wherein the voltagebreakdown means is a Zener diode that has a breakdown voltage that is equal to or greater in magnitude than the magnitude of the voltage of the triggering signal.

13. The invention of claim 12 wherein the electrically resistive elements are thermal printing elements.

14. A electrical circuit comprising:

(a) a plurality of triggerable switching devices that are reversibly switchable between an initial high-impedance state and a low-impedance state which have a first load electrode and a second load electrode, each switching means being assigned to one group of a plurality of groups of switching means-electrically resistive element combinations, and

(b) a plurality of electrically resistive elements, each being assigned to one group of said combinations and each having a first terminal that is connected to the second load electrode of a switching means and a second terminal that is connected in common with the second terminal of the other electricall resistive elements of a group of said combinations, and

(c) means to successively apply a plurality of sets of selective triggering signals to the first load electrodes of the switching means, the plurality of sets of selective triggering signals being equal in number to the number of groups of said combinations, and

(d) means to apply a supply voltage pulse to the first load electrodes of those switching means which have no simultaneous triggering signal applied thereto during the time that the plurality of sets of triggering signals are being applied to the switching means, and

(e) means to apply a first potential to the common second terminals of the electrically resistive elements of each group of said combinations in an individual manner and to simultaneously apply a second potential to the common second terminals of all of the other electrically resistive elements, the first potential being applied to the common second terminals of the electrically resistive elements of a group of said combinations during the time that a set of selective triggering signals are being applied to the first load electrodes of the switching means, the means to apply the first and the second potentials being constructed so as to provide a current path that allows at least a predetermined minimum amount of electrical current to flow through the switching means due to the application of the supply voltage pulse, the application of a selective triggering signal being effective to switch a selected switching means in the group of said combinations which has the first potential applied thereto from an initial high-impedance state to a low-impedance state, the termination of the supply voltage pulse being elfective to switch the previously selected switching means back to the high-impedance state.

15. The invention of claim 14 wherein the electrically resistive elements are thermal printing elements.

16. The invention of claim 14 wherein the switching means are two-terminal glass body switching devices.

17. The invention of claim 14 wherein the switching means are two-terminal semiconductive switching devices.

18. The invention of claim 14 wherein the means to apply the supply voltage pulse includes a plurality of isolation means, each isolation means being associated with a particular individual supply line and being constructed to block the application of a supply voltage pulse to the particular individual supply line when a triggering signal is simultaneously applied to the individual supply line, and to allow said supply voltage pulse to be coupled through the individual supply line to the switching means when a triggering signal is not simultaneously applied to the individual supply line.

.19. The invention of claim 14 wherein the means to apply the first and the second potentials comprises:

(a) a plurality of group selection transistors each having a control electrode and a first and a second load electrodes, each transistor having its first load electrode connected to all of the first junctions of one group of said combinations, and

(b) a selective energization signal means that is coupled to apply an energization signal which is effective to saturate a group selection transistor to the control electrodes of each of the group selection transistors in a successive and individual manner, and

(c) a plurality of voltage-breakdown means, each voltage-breakdown means being connected across the second and the first load electrodes of a separate group selection transistor, so as to provide a substantially constant voltage across the transistor when the transistor is cutoff, the voltage-breakdown means having a voltage-current characteristic such that the current flowing through an associated switching means predominantly flows through the transistor when the transistor is saturated and predominantly flows through the voltage-breakdown means when the transistor is cut-off, and

(d) a resistor connected between the first load electrode of each group selection transistor and a power supply.

20. The invention of claim 19 wherein the voltagebreakdown means is a Zener diode and the magnitude of the voltage of the triggering signal minus the magnitude of the Zener voltage of the Zener diode is greater than the forward breakover voltage of the switching device.

21. The invention of claim 16 wherein the supply voltage pulse includes a plurality of isolation means, each isolation means being associated with a particular individual supply line and being constructed to block the application of a supply voltage pulse to the particular individual supply line when a triggering signal is simultaneously applied to the individual supply line, and to allow said supply voltage pulse to be coupled through the individual supply line to the switching means when a triggering signal is not simultaneously applied to the individual supply line.

22. The invention of claim 21 wherein the means to apply the first and the second potentials comprises:

(a) a plurality of group selection transistors each having a control electrode and a first and a second load electrodes, each transistor having its first load electrode connected to all of the first junctions of one group of said combinations, and

(b) a selective energization signal means that is coupled to apply an energization signal which is effective to saturate a group selection transistor to the control electrodes of each of the group selection transistors in a successive and individual manner, and

(c) a plurality of voltage-breakdown means, each voltage-breakdown means being connected across the second and the first load electrodes of a separate group selection transistor so as to provide a substantially constant voltage across the transistor when the transistor is cut-off, the voltage-breakdown means having a voltage-current characteristic such that the current flowing through an associated switching means predominantly flows through the transistor when the transistor is saturated and predominantly flows through the voltage-breakdown means when the transistor is cut-off, and

(d) a resistor connected between the first load electrode of each group selection transistor and a power supply.

23. The invention of claim 22 wherein the voltagebreakdown means is a Zener diode and the magnitude of the voltage of the triggering signal minus the magnitude of Zener voltage of the Zener diode is greater than the forward-breakover voltage of the switching device.

24. The invention of claim 23 wherein the electrically resistive elements are thermal printing elements.

25. The invention of claim 17 wherein the means to apply the supply voltage pulse includes a plurality of isolation means, each isolation means being associated with a particular individual supply line and being constructed to block the application of a supply voltage pulse to the particular individual supply line when a triggering signal is simultaneously applied to the individual supply line, and to allow said supply voltage pulse to be coupled through the individual supply line to the switching means when a triggering signal is not simultaneously applied to the individual supply line.

26. The invention of claim 25 wherein the means to apply the first and the second potentials comprises:

(a) a plurality of group selection transistors each having a control electrode and a first and a second load electrodes, each transistor having its first load electrode connected to all of the first junctions of one group of said combinations, and

(b) a selective energization signal means that is coupled to apply an energization signal which is effective to saturate a group selection transistor to the control electrodes of each of the group selection transistors in a successive and individual manner, and

(c) a plurality of voltage-breakdown means, each voltage-breakdown means being connected across the second and the first load electrodes of a separate group selection transistor so as to provide a substantially constant voltage across the transistor when the transistor is cut-off, the voltage-breakdown means having a voltage-current characteristic such that the current flowing through an associated switching means predominantly flows through the transistor when the transistor is saturated, and predominantly flows through the voltage-breakdown means when the transistor is cut-off, and

(d) a plurality of resistors, each resistor being connected between the first load electrode of each group selection transistor and a power supply.

27. The invention of claim 26 wherein the voltagebreakdown means is a Zener diode and the magnitude of the voltage of the triggering signal minus the magnitude of the Zener voltage of the Zener diode is greater than the forward-breakover voltage of the switching device.

28. The invention of claim 27 wherein the electrically resistive elements are thermal printing elements.

References Cited UNITED STATES PATENTS 2,917,996 12/1959 Epstein et al.

3,139,026 6/1964 Meckstroth et a1. 34676 X 3,323,241 6/1967 Blair et al 219-543 X 3,354,817 11/1967 Sakurai et al.

3,409,902 11/1968 Merryman 219-216 JOSEPH V. TRUHE, Primary Examiner P. W. GOWDEY, Assistant Examiner US. Cl. X.R. 34676 

